This technology is directed to a method for differentiating fertile and sterile plant lines at the DNA level, by detecting polymorphisms in chloroplast DNA.
Hybrid vigor in crop plants has been recognized as a widespread and powerful tool to increase yields. Commercial hybrids are used extensively in many crops, including corn, sorghum, sugarbeet, colza, wheat, and sunflower.
Commercial hybrids have the greatest potential for crops when the hybrid seed can be produced reliably and economically. The three biological requirements for successful hybrid seed production are: (a) the presence of hybrid vigor; (b) elimination of fertile pollen in the inbred line that will act as female parent; and (c) adequate pollination by the inbred line that will act as male parent. If these biological requirements are met for a particular species, a practical program of seed production on a large scale may be developed to produce hybrids for use by farmers.
The second of the aforementioned requirements—the elimination of fertile pollen in a line that will act as female parent of the hybrid—has led scientists to endeavor to understand the biological mechanisms of obtaining male sterility in a hermaphrodite species. Accordingly, several procedures or systems were developed to obtain male sterility, including genetic male sterility, cytoplasmic male sterility, and nuclear-cytoplasmic male sterility. Nuclear-cytoplasmic male sterility, in particular, is widely used for commercial seed production of hybrid seed in hermaphrodite crop species (Wright, 1980).
Three types of inbred lines must be developed and maintained when employing a cytoplasmic-genetic system to produce hybrids, as in the case of hybrid corn, sorghum, pearl millet, sunflower, and other species. In this system, the female parent harbors factors in the cytoplasm of cells, which render it male-sterile in the absence of appropriate restorer genes in the nucleus. The cytoplasm of the female gamete, but not that of the male, is transmitted to the offspring, thereby making it possible to produce female populations which are free of any male-fertile individuals (Forsberg & Smith, 1980).
The female parent in the cytoplasmic genetic system must have two different, but phenotypically identical forms. One form is male sterile, with sterile cytoplasm (S) and with nonrestoring genes (rfrf) in the nucleus. The other form is male fertile, with normal, fertile, cytoplasm (N) and also with nonrestorer genes in the nucleus (rfrf). The male sterile version is referred to as the cytoplasmic male sterile (cms) line, the A line, or the sterile line. The normal male fertile version is called the maintainer line or the B line. Generally, the cms line is developed by crossing a normal male fertile line, as male, to a male sterile female with sterile cytoplasm and nonrestoring genes, followed by repeated backcrossing to the normal male fertile line. After the cms line has been developed, a cms line seed is produced for line maintenance by crossing the sterile cms line (S, rfrf) with the normal male fertile line (N, rfrf). All offspring from the cross will be male-sterile. The normal male fertile lines (or maintainer lines or B lines) are maintained by a routine selfing, sibbing, or open pollination in isolation.
The third type of inbred line in the cytoplasmic-genetic system is the restorer male parent. Accordingly, it must have the homozygous dominant genotype for fertility restoration (RfRf). These restorer lines, or R lines, can be developed by using a line with restorer genes as the donor parent and the genotype (line) desired for use as a restorer line as the recurrent parent in a series of backcrosses. It is very convenient if a donor parent also has sterile cytoplasm (S, RfRf) and is used as the female, because the presence of the restorer allele in segregating populations after each backcross is phenotypically apparent. The restorer male parent lines are fertile and are maintained by routine selfing, sibbing or open pollination in isolation.
For hybrid seed production, rows of the cms line are interplanted with rows of the fertility restorer line in ratios from 2:1 to 6:1. Hybrid seeds from the cross cms×restorer, are then harvested on the cms line.